The prior art comprises different types of touch-sensitive systems that operate by transmitting light inside a solid light transmissive panel, which is defined by two parallel boundary surfaces connected by a peripheral edge surface. Specifically, light is injected into the panel so as to propagate by total internal reflection (TIR) between the boundary surfaces. An object that touches one of the boundary surfaces (“the touch surface”) causes a change in the propagating light that is detected by one or more light sensors. In one implementation, e.g. as disclosed in WO2008/017077, US2009/267919 and WO2010/056177, light sensors are arranged behind the panel to detect light which scatters off the touching object and escapes the panel via the boundary surface opposite to the touch surface. In another implementation, e.g. as disclosed in U.S. Pat. No. 7,435,940, light sensors are arranged at the periphery of the panel to detect light which scatters off the touching object and is confined within the panel by total internal reflection. In yet another implementation, e.g. as disclosed in WO2010/006882 and WO2010/134865, light sensors are arranged at the periphery of the panel to sense the attenuation of the light transmitted through the panel.
In order to achieve a uniform illumination of the touch surface from within the panel, the incoupling site is often elongate and extends along a significant portion of the panel. Most prior art documents propose injecting the light through an elongate portion of the peripheral edge surface, without any dedicated coupling elements. Such an approach is possible since the light can be injected at a relatively steep angle to the edge surface, resulting in comparatively small reflection losses at the edge surface. Also, such an incoupling site does not add significantly to the thickness of the touch system. However, incoupling via the edge surface requires the edge surface to be highly planar and free of defects. This may be difficult and/or costly to achieve, especially if the panel is thin and/or manufactured of a comparatively brittle material such as glass. Incoupling via the edge surface may also add to the footprint of the touch system. Furthermore, it may be difficult to optically access the edge surface if the panel is attached to a mounting structure, such as a frame or bracket, and it is also likely that the mounting structure causes strain in the edge surface. Such strain may affect the optical quality of the edge surface and result in reduced incoupling performance.
Above-mentioned WO2010/006882 and WO2010/134865 propose incoupling of light via elongate wedges that are attached and optically coupled (glued) to the top or bottom surfaces. Such an approach may mitigate any strict requirements for the surface properties of the edge surface and/or facilitate mounting of the panel. However, in order to achieve a uniform illumination of the touch surface by propagating light that is collimated in the depth direction of the panel, the incoupling site needs to admit a beam of light with such an extent in the depth direction of the panel that the footprint of the beam on the touch surface essentially overlaps between successive reflections in the touch surface. This, in turn, means that the wedge needs to have a light-receiving surface of corresponding dimensions, resulting in a wedge that may need to project at least 15-20 mm from the top or bottom surface. Such a wedge may add significant thickness and weight of the system. To reduce weight and cost, the wedge may be made of plastic material. On the other hand, the panel is often made of glass, e.g. to attain required bulk material properties (e.g. index of refraction, transmission, homogeneity, isotropy, durability, stability, etc) and surface evenness of the top and bottom surfaces. The present applicant has found that the difference in thermal expansion between the plastic material and the glass may cause the wedge to come loose from the panel as a result of temperature variations during operation of the touch system. Even a small or local detachment of the wedge may cause a significant decrease in the performance of the system.
The present applicant has tried to overcome this problem by attaching several shorter wedges side-by-side so as to form the elongate incoupling site. However, if the touch system requires more than one sheet of light to be injected via the incoupling site, such that the light transmitted via the incoupling site has more than one main direction in the plane of the panel (i.e. as seen in a plan view of the touch surface), the joints between the wedges may interfere with (e.g. reflect) the incoming light and cause a significantly reduced performance of the system. For example, WO2010/006882 and WO2010/134865 disclose techniques for enabling multi-touch sensitivity by injecting plural sheets with different main directions via the incoupling site.
The prior art also comprises US2004/0252091, which discloses an optical touch system in which diverging light beams are coupled into a light transmissive panel for propagation by TIR via large wedges in the form of revolved prisms that are arranged on the top or bottom surface of the panel.
Outside the field of optical coupling elements for touch systems, it is known to provide flat panel displays with a so-called brightness enhancement film (BEF), which is a transparent optical film designed to increase the display brightness through improved light management, see e.g. US2005/248848 and US2010/259939. Specifically, the BEF is a micro-structured sheet with a plurality of prismatic and/or lenticular elements and may be adhered to a light-transmissive substrate in the display. The micro-structures are designed to increase the spatially averaged luminance of the display in a certain range of angles around a normal (perpendicular) viewing direction.
U.S. Pat. No. 6,972,753 discloses an optical touch panel in which a BEF-type element (“a prism lens sheet”) is attached to a light source arranged alongside a light guide panel, so as to enhance the directivity of the emitted light before the light is directed onto the edge surface of the light guide panel. The prism lens sheet is not a coupling element, but rather an upstream collimator that ensures that all light is injected in a single, well-defined main direction in the plane of the panel.
U.S. Pat. No. 6,803,900 discloses a lighting system for an LCD display. The lighting system comprises a side-illuminated flat light guide with micro-optical structures on its top surface that give a preferential outcoupling of light. A light pipe is disposed parallel to the light guide to couple light into the light guide through its peripheral edge surface. The light pipe is provided with micro-optical surface structures which cause light, which is guided inside the light pipe from one end towards the other end, to be re-directed towards the peripheral edge surface of the light guide.
WO2007/112742 discloses an optical touch pad in which a beam expander is arranged intermediate an emitter of collimated light and the edge surface of a light transmissive panel. A beam splitter in the form of a plurality of prisms is formed on the edge surface to receive the expanded beam and divide it into two expanded collimated beams with controlled angles of incidence inside the panel. The beam splitter is only useful on the edge surface since it is designed to produce two beams with different directions in the depth direction of the panel.